Stimulation of Host Resistance to Metastatic Tumors by Macrophage Activating Agents Encapsulated in Liposomes
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The disappointing results obtained in experimental and clinical efforts to devise effective, specific, active immunotherapy procedures for the treatment of cancer have stimulated renewed interest in mechanisms of non-specific “natural” antitumor surveillance mediated by macrophages and natural killer cells. A significant effort is now underway in many laboratories to develop effective biological response modifier (BRM) agents that can stimulate the antitumor activities of these cells. Liposomes offer a useful carrier system for delivering BRM agents to macrophages in vivo. When injected i.v. the majority of liposomes are taken up by phagocytic reticuloendothelial cells in the liver and spleen, and by circulating monocytes (reviewed in 6). The passive localization of liposomes into mononuclear phagocytes is frustrating to investigators who wish to target liposomes to other cell types in the body, including tumor cells, but provides a highly effective mechanism for “targeting”, albeit passively, of liposome-encapsul-ated materials to macrophages. We have exploited this pathway to deliver natural and synthetic molecules with macrophage activating activity to macrophages in situ.
KeywordsTumor Immunity Biological Response Modifier Muramyl Dipeptide Active Immunotherapy Median Life Span
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